/Engine/Source/Runtime/Core/Public/Misc/MapErrors.h
Run 'Clean BSP Materials' to clear {count} references: This warning indicates that there are material references on brush faces that aren't contributing to the BSP, and that applying the Tools->'Clean BSP Materials' operation can clean up these references.
static [FLazyName](API\Runtime\Core\UObject\FLazyName) CleanBSPMaterials;